prominent, filiform, sometimes obsolete anteriorly, and 

 behind the middle are seven or eight fornicated costae ; 

 concentric wrinkles more obvious and somewhat undu- 

 lated on the anterior margin ; lunule ovate-acute, simply 

 sculptured with the concentric wrinkles ; ivithin radiated 

 with strongly indented lines, which are obsolete on the 

 anterior margin ; teeth two in each valve, the posterior one 

 of the right valve sometimes so deeply divided as to re- 

 semble two, and one of those of the left valve rudimental. 



SYNONYMS. 



V. PHOLADiFORMis,Z/am. •5w. «aw5 Vertebr.,\o\. 5. p. 505. 

 Sower by' s Genera, pi. Petricola, fig. 1 and 2. Conrad's 

 Marine shells, pi. 7. Desh. Enc. Meth. p. 747. 



P. FORNicATA, S. Joum. Acad. Nat. Sc. vol. 2, p. 319. 



OBSERVATIONS. 



This shell may be truly said to be an extraordinary spe- 

 cies, having the deceptive exterior aspect of a Pholas, and 

 like many of that genus residing in cavities drilled out of 

 the most compact earth, as is also sometimes the case with 

 the P. ochroleuca. Lam. 



It is abundant on many parts of our coast from Maine 

 to Florida. Dr. Ravenel sent me specimens from Charles- 

 ton and Dr. Storer informs me that it is common at Chel- 

 sea and Martha's Vineyard in Massachusetts. 



RKl'ERENCF, TO THE PLATE. 



The upper and lowor figures, ropresont the exterior and dorsal views. 



